1. Field of the Invention
The present invention relates to a transfer sheet for electrophotography, particularly, to a transfer sheet for electrophotography which causes no mottles in the recorded image portion and provides an image with a high quality small in gloss contrast between the blank portion and the image portion, when used in a full color type or monochromatic copying apparatus of indirect dry-type electrophotography or in a printer.
2. Description of the Related Art
Intensive studies have been made in an attempt to improve the quality of the recorded picture image in electrophotography in accordance with recent trends toward the coloring and digital mode operation of the electrophotographic copying apparatus or printer. Particularly, in a full color copying apparatus and printer of electrophotographic system, a digital processing for input/output of a picture image is widely employed nowadays in order to obtain a high quality picture image. Along this line, marked improvements have been made in the picture image input method, processing method of the input picture image, developing method, transfer method, fixing method, etc. Marked improvements have been also made in the image forming materials including the developing agents and the photosensitive agents in accordance with advent of digital recording of high precision and color recording of a high color density.
However, when the conventional transfer sheets for electrophotography are used in the full color copying apparatus or printer of the improved electrophotographic system described above, there are problems, for example, in that the clarity of picture image is impaired by the disturbance of mottles and dots in the region ranging from the half tone portion to the high density picture image portion included in the solid picture image and in that the gloss feel in the high density picture image portion is rendered excessive and in that, accordingly, the difference in gloss between the high density picture image portion and the half tone portion or blank portion is rendered prominent. As a result, the entire picture image gives a highly unnatural impression.
Methods for suppressing the generation of picture image defects such as blister in the recording by coated type transfer sheet when used in indirect dry type electrophotography have been proposed in, for example, Japanese Patent Disclosure (hereinafter referred to as "JP Kokai") Nos. 62-198877 and 3-294600, which methods comprising keeping the air permeability of a smooth coated paper sheet at a certain level or lower, or adding a non-film forming resin to the transfer sheet, so as to improve the recorded picture image quality and eliminate the picture image defects. It has been also proposed in JP Kokai No. 62-198877 that unsatisfactory image transfer under high humidity can be improved by maintaining the surface electrical resistance at a certain level or higher under high humidity. Further, it has been also proposed in JP Kokai No. 3-242654 that unsatisfactory image transfer under high humidity can be improved by using a special emulsion type adhesive. However, these prior art techniques are insufficient in improvements of the defects such as the mottle generation in the picture image portion and the unnatural gloss in the high density picture image portion.
In recent years, demands for high image quality have become much severer in, for example, the full color copying apparatus. In particular, severer demands are being directed to improvements of defects such as the mottle generation in the region ranging from the half tone portion to the high density picture image portion and the gloss contrast between the blank portion and the picture image portion.